The present invention relates to a process for the preparation of polymer melts which are substantially free of volatile components utilizing a starting material of highly viscous alkene polymer melts containing solvent(s) and/or monomer residues.
It is known to use screw extruders provided with degasification orifices at reduced pressure for the degasification of solvent-containing polymer melts. However, in conventional multi-screw extruders in particular, the processing of certain alkene polymer melts causes such increased heating of the polymer melt that cooling through the extruder wall is necessary.
However, it has been discovered that using such conventional degasification extruders for the processing of highly viscous polymer melts can have unfavorable consequences. Indeed, it has been found that when alkene polymers are processed having a melt index (as will be defined hereinafter) of less than 4 dg/min, under the usual cooling conditions, the polymer melt in the degasification zone(s) of the extruder balls into a mass which no longer spreads over the extruder screw. This phenomenon has an extremely adverse effect on the degasification of the polymer melt.
It has now been found that such disadvantageous phenomena occurring when highly viscous polyalkene melts are processed in a temperature-controlled degasification extruder can be avoided and that polymer melts can be obtained which are substantially free of volatile components by working under well-defined conditions according to the present invention.
These new processing conditions according to the present invention are characterized in that an alkene polymer melt having a melt index lower than 4 and at a content of volatile components of less than 10 percent, is treated in one or more sections incorporated in a cooled degasification extruder having at least one kneading zone where the melt is subjected to pressure buildup and substantially, at least one degasification zone where the melt is subjected to reduced pressure. The cooling in the kneading zone(s) according to the present invention is intense, whereas in the degasification zone(s) cooling or heating is less intense.
Generally, such a temperature control, or rather heat exchange, cannot be effected straightforwardly. An extruder is usually surrounded by a jacket enabling heating or cooling of the contents of the extruder with a heating or cooling liquid or with other media for the supply or withdrawal of heat. Such a jacket wall is usually divided into 3 or 4 zones so as to enable independent temperature settings at the feed section, in two or more zones along the screw or screws and at the head. For the realization of the process according to the invention it is necessary that the temperatures in the kneading zones and in the degasification zones of the extruder to be used can be set independetly of each other, or rather that the heat exchange, in particular the withdrawal of heat, can be controlled independently in these zones. This requires that the extruder be provided with suitable contrivances. This means that the jacket wall must be divided into section coinciding with the kneading zones and the degasification zones, in which sections different cooling rates can be effected, for example by a difference in temperature between the cooling liquids. Any other known method can of course also be used.
By the term "melt index" of an alkene polymer as used herein, it is intended to refer to the amount of polymer flowing through a standard orifice under standard conditions of temperature and pressure, expressed in decigram/minute (dg/min). For polyethylene, in particular, the standard conditions according to ISO standard R 292 resp. ASTM standard D 1238 condition E are used. For other alkene polymers, the standards used are such that product with processing and flow characteristics which, under adapted temperatures and pressure conditions, are comparable to those of particular polyethylene products, and which exhibit a melt index comparable to the melt index of such particular polyethylene products according to the ISO standard R 292 and ASTM standard D 1238. For example, at a load of 2.16 kg and a temperature of 503.degree. K. such other alkene polymers yield melt index values for polypropylene which, in practice, are comparable to the melt index values according to ISO standard R 292 resp. ASTM D 1238 for polyethylene.
Further research has shown that the "balling" effect of the polymer melt occurs when the differential in temperature between the polymer melt and the cooled extruder wall exceeds a particular critical value. On the one hand, this critical difference in temperature appears to depend on the viscoelastic properties (which may be influenced by e.g., molecule structure) of the polyalkene processed, the viscoelastic properties as such depending on the process temperature. Thus, it has been observed that the higher the molecular weight of the material, and/or the more branched its chains, the smaller the critical difference in temperature between the polymer melt and the extruder wall which marks incipient balling. In addition, the critical difference in temperature is smaller the lower the temperature is for the melt is lower.
Preferably, the process according to the present invention is embodied so that in a cooled degasification extruder, the temperature of the extruder wall in the degasification zone(s) is kept higher than the temperature at which the polymer melt would ball into a mass thereby wholly or partially unsticking from the extruder screw at the relative polymer melt composition and temperature.
Thus, the temperature of the extruder jacket in the degasification zone(s) should satisfy the relation: EQU T.sub.ext &gt;T.sub.m -.DELTA.T.sub.cr
where T.sub.ext is the temperature of the interior wall of the jacket of the extruder, T.sub.m is the temperature of the polymer melt, and .DELTA.T.sub.cr refers to the characteristic difference in temperature between a cooled extruder wall and the polymer melt which marks the balling of the polymer melt at the relative composition and temperature of the polymer melt. The characteristic extruder wall temperature can easily be determined empirically by one skilled in the art for any composition of polymer desired to be processed.
The efficiency of the process according to the present invention is improved when a stripping agent for the volatile components is kneaded into the polymer melt in the kneading zone, such as, in particular, water and/or steam, methanol or an inert gas, for example, nitrogen.
As mentioned above, in the process according to the present invention the polyalkene polymer is processed in the degasification zone(s) at reduced pressure, the content of volatile components being lower than 10% by weight. However, the process is particularly suited for the processing of polymer solutions as obtained in processes by which alkenes are (co)polymerized in solution (the so-called solution processes), which solutions have a content of volatile components (solvent, such as, for example, cyclohexan, hexane or gasoline, and comonomer, such as for instance hexene, octene or decene) that generally is considerably higher.
According to one embodiment of the present invention, a polymer solution having a content of volatile components of between about 5 to about 25 wt. percent can be initially passed through a flash evaporator zone in the extruder with backward flash degasification where the content of volatile components is reduced to less than about 2-10 wt. percent before the low-pressure degasification zone(s).
Preferably, a devolatilization extruder is used which has one backward flash degasification orifice and two forward low-pressure degasification zones, the pressure at the backward degasification orifice being approximately atmospheric, whereas in the first degasification zone a pressure ranging between about 50 and about 250 mbar is maintained and in the second degasification zone, a pressure ranging from between about 10 and about 100 mbar is maintained. Suitably, use can be made of a twin-screw extruder with co-rotating screws, and in particular, a twin-screw extruder with parallel, fully intermeshing co-rotating screws.
According to another preferred embodiment of the present invention, the efficiency of the process can be improved by using a gear-type pump for the eventual pressure buildup in the polymer melt from the last degasification zone to the die plate. Indeed, using such a gear-type pump enables degasification at a higher temperature, without too high of a temperature which can be disadvantageous to the polymer developing at the die plate.
The process according to the present invention is particularly suitable for the treatment of polyalkene melts consisting of an ethylene polymer or copolymer with a melt index lower than 4 according to ISO standard R 292, resp. ASTM D 1238 and, in particular, with a melt index lower than 3. Materials which are especially suitable for processing according to the present inventive process are, in particular, polyalkene melts of so-called linear low-density polyethylene.